Auscultation is defined as the act of listening for sounds made by a patient's internal organs (e.g., the heart and lungs) to aid in the diagnosis of certain disorders. Typically, auscultation is practiced using a standard stethoscope by a healthcare professional to count respiration rate and listen for lung and heart function. However, using a stethoscope over a long period of time is impractical and inaccurate due primarily to human error. To more accurately count respiration rate, a technique called phonopneumography is used to record and analyze breath sounds measured using an acoustic sensor. Specifically, the acoustic sensor detects analog acoustic signals associated with respiration; these signals can then be digitized and analyzed using a computer algorithm to derive respiratory rate.
A number of issued U.S. Patents describe respiration rate calculated using phonopneumography. For example, U.S. Pat. Nos. 6,261,238 and 5,309,922 both describe an apparatus that includes conventional acoustic sensors and processing components that use this technique to determine respiration rate.
PTT, defined as the transit time for a pressure pulse launched by a heartbeat in a patient's arterial system, has been shown in a number of studies to correlate to both systolic and diastolic blood pressure. In these studies, PTT is typically measured with a conventional vital signs monitor that includes separate modules to determine both ECG and pulse oximetry. PTT is typically defined as the temporal difference between a portion of the time-dependent ECG waveform, which is typically measured with electrodes, and a portion of a time-dependent optical waveform (called a photoplethysmograph, or PPG), measured with a pulse oximeter.
Specifically, during a PTT measurement, multiple electrodes typically attach to a patient's chest to determine a time-dependent ECG component featuring a sharp spike called the R-wave of a ‘QRS complex’. This feature indicates an initial depolarization of ventricles within the heart and, informally, marks the beginning of the heartbeat and a pressure pulse that follows. Pulse oximetry is typically measured with a bandage or clothespin-shaped sensor that attaches to a patient's finger, and includes optical systems operating in both the red and infrared spectral regions. A photodetector measures radiation emitted from the optical systems and transmitted through the patient's finger. Other body sites, e.g., the ear, forehead, and nose, can also be used in place of the finger. During a measurement, a microprocessor analyses both red and infrared radiation measured by the photodetector to determine the patient's blood oxygen saturation level and the PPG. Time-dependent features of the PPG indicate both pulse rate and a volumetric, optical absorbance change in an underlying artery (e.g., in the finger) caused by the propagating pressure pulse.
A number of issued U.S. Patents describe the relationship between PTT and blood pressure. For example, U.S. Pat. Nos. 5,316,008; 5,857,975; 5,865,755; and 5,649,543 each describe an apparatus that includes conventional sensors that measure an ECG and the PPG, which are then processed to determine PTT.